1. Field of the Invention
The present invention relates to a disk drive for writing/reading data on/from a disk-shaped storage medium such as an optical disk and also to a method of detecting pre-pits.
2. Description of the Related Art
In order to write data on a disk, it is required that the disk has guide means for forming data tracks. To meet the above requirement, grooves serving as pre-grooves are formed and resultant grooves or lands (protrusions having the shape of plateau in cross section between adjacent grooves) are used as data tracks.
In order to write data at a desired location on a data track, it is required that an address information be recorded on the disk. The address information is generally recorded by wobbling the grooves or by forming pre-pits on the data tracks.
For example, in the case of DVD-RW that is a rewritable version of DVD (Digital Versatile Disc) based on the phase change recording or DVD-R that is a write-once disk using an organic dye material, wobbling grooves G are formed as pre-format on the disk and land pre-pits LPP are formed on lands L between adjacent grooves G, as shown in FIG. 6.
In this specific case, reflected-light information provided by the wobbling grooves is used to control the rotation of the disk and also used to produce a master clock signal used in writing data. The land pre-pits are used to determine precise write locations of respective bits and also used to acquire various kinds of disk information such as pre-address information. That is, addresses indicating the physical locations on the disk are recorded using land pre-pits LPP.
A disk drive adapted to such a disk reads addresses by detecting land pre-pits formed on the disk and performs various controls in the writing/reading operation on the basis of the detected pre-pit information indicating the location on the disk.
FIG. 7 shows a format of land pre-pits LPP.
Each interval of a track including 8 wobbles forms one frame, and each set of one even-numbered frame and one odd-numbered frame, including a total of 16 wobbles, forms one unit of land pre-pit information.
As shown in FIG. 6, land pre-pits LPP are formed by forming cutouts in the lands in synchronization with wobbles. One bit of address data is expressed by one set of land pre-pits LPP.
FIG. 7A shows an example in which land pre-pit information is formed in even-numbered frames. In this case, first 3 wobbles of each even-numbered frame form one set of land pre-pits LPP.
Let b2, b1, and b0 represent the presence/absence of land pre-pits LPP. If (b2, b1, b0) is (1, 1, 1), that is, when three land pre-pits are formed, a set of those pre-pits LPP serves as a sync signal. A data bit of “1” is expressed by forming two land pre-pits LPP at b2 and b0. That is, when (b2, b1, b0)=(1, 0, 1), the data bit is “1”. On the other hand, a data bit of “0” is expressed by forming one pre-pit LPP at b2. That is, when (b2, b1, b0)=(1, 0, 0), the data bit is “0”.
FIG. 7B shows an example in which land pre-pit information is formed in odd-numbered frames. In this case, first 3 wobbles of each odd-numbered frame form one set of land pre-pits LPP whose absence/presence are expressed by (b2, b1, b0).
In the case in which land pre-pit information is formed in odd-numbered frames, when (b2, b1, b0)=(1, 1, 0), a set of those land pre-pits represents a sync signal. As in even-numbered frames, a data bit of “1” is represented by (b2, b1, b0)=(1, 0, 1), and a data bit of “0” is represented by (b2, b1, b0)=(1, 0, 0).
In FIG. 7C, sync signals and data bits represented by combinations of b2, b1, and b0 are summarized in the form of a table.
In each 16-wobble interval, land pre-pits LPP are formed only in either an even-numbered frame or an odd-numbered frame. Determination as to in which frame to form land pre-pits LPP is made for each 16-wobble interval such that land pre-pits LPP are not formed on both adjacent groove tracks on a disk.
Information expressed by land pre-pits LPP can be acquired in the form of a push-pull signal by detecting light reflected from a disk. More specifically, the push-pull signal is obtained as a differential signal between signals corresponding to the intensities of light reflected from a left-hand part and a right-hand part of a laser spot scanning on the disk in a track line direction.
FIG. 8 shows a circuit for detecting land pre-pits LPP.
The disk drive has an optical head including a photodetector 51, such as a quadrant photodetector having four photodetector elements A, B, C, and D, for detecting light reflected from a disk.
In this specific case, signals output from the photodetector elements A and C of the photodetector 51 are added together by an adder 56, and signals output from the photodetector elements B and D are added together by an adder 55. The outputs of the adders 55 and 56 are supplied to a push-pull signal generator 52. The push-pull signal generator 52 includes a differential amplifier A1 and resistors R11 to R14.
The push-pull signal generator 52 outputs a push-pull signal P/P proportional to ((A+C)−(B+D)).
In the push-pull signal P/P, as shown in FIGS. 9A and 9B, relatively large amplitudes (SLP1, SLP2, and SLP3) corresponding to land pre-pits LPP are obtained. Thus, information represented by land pre-pits LPP can be detected by detecting the large amplitudes.
To achieve the above, a reference voltage Vth is supplied from a reference voltage source 54 to a comparator 53, and the comparator 53 compares the push-pull signal P/P with the supplied reference voltage Vth. The comparator 53 outputs a two-level signal indicating the comparison result. Thus, a detection signal LPPout corresponding to the land pre-pits LPP is obtained.
High and low levels of this detection signal LPPout corresponding to the land pre-pits LPP correspond to “1” and “0”, respectively, of b2, b1, and b0 of the land pre-pits LPP.
Furthermore, a decoder (not shown) extracts address information by detecting sync signals and data bits (with a level of “1” or “0”) corresponding to b2, b1, and b0.
FIGS. 9A and 9B show land pre-pit detection signals LPPout obtained by comparing the push-pull signal P/P with the threshold voltage Vth.
A known technique of detecting land pre-pits LPP may be found, for example, in U.S. Pat. No. 6,337,838.
However, if information is written on grooves serving as recording tracks, recording marks (phase change bits) formed on the grooves interfere with the land pre-pits LPP. As a result, it becomes difficult to correctly read the land pre-pits LPP. More specifically, the interference of recording marks results in a reduction in reflectance, and thus a reduction occurs in amplitude of the push-pull signal P/P corresponding to the land pre-pits LPP.
The push-pull signal P/P has amplitude variations due to wobbling of tracks, crosstalk from adjacent tracks, and a variation in quality of the disk.
The influence of such variations in amplitude of the push-pull signal P/P on the detection of pre-pits is described below with reference to FIGS. 9A and 9B.
By way of example, let assume that three pulses corresponding to three land pre-pits LPP ((b2, b1, b0)=(1, 1, 1)) indicating a sync signal in an even-numbered frame appear in the push-pull signal P/P.
In the example shown in FIG. 9A in which three pulse components SLP1, SLP2, and SLP3 appear in the push-pull signal P/P, the amplitude of the third pulse component SLP3 is smaller than the amplitudes of the first and second pulse components SLP1 and SLP2.
The reduction in the amplitude SLP3 is caused by presence of a recording mark M adjacent to the land pre-pit LPP, as represented by i in FIG. 6.
As can be seen from the envelope of the waveform of the push-pull signal P/P shown in FIG. 9A, the push-pull signal P/P has a periodic variation in level caused by wobbles. The push-pull signal P/P also includes a variation in level due to crosstalk noise.
As described above with reference to FIG. 8, when the land pre-pit detection signal LPPout is produced by comparing the push-pull signal P/P with the threshold voltage Vth, if the threshold voltage Vth is set to a level as shown in FIG. 9A, the third land pre-pit LPP (pulse SLP3) is not detected.
That is, (b2, b1, b0)=(1, 1, 1) is erroneously detected as (b2, b1, b0)=(1, 1, 0).
If the threshold voltage Vth is reduced, correct detection is possible even for pulse components corresponding to land pre-pits LPP whose amplitude is reduced by the presence of recording marks. However, the reduction in the threshold voltage can cause a component of the push-pull signal P/P, which does not correspond to a pre-pit LPP but whose amplitude is increased by wobbling or noise, to be erroneously detected as a pre-pit in the detection signal LPPout, as is the case with incorrect pulses denoted by N in FIG. 9B.
As described above, the amplitude variation of the push-pull signal P/P due to wobbling and/or recording marks can cause the problem that land pre-pits LPP are not correctly detected.
Incorrect detection of land pre-pits LPP results in an increase in address error rate. That is, it becomes impossible to correctly read address information. This results in degradation in performance of operation of writing/reading data on/from a disk and also degradation in seeking operation.
U.S. Pat. No. 6,337,838 cited above discloses a technique of reducing the variation in amplitude of the push-pull signal P/P by using a so-called AGC circuit. However, U.S. Pat. No. 6,337,838 does not disclose a technique of properly setting a threshold value used in producing the land pre-pit detection signal LPPout.